As taught by the prior art, it is often desirable in the field of graphic arts to effect the cure of UV curable organic materials at wave lengths greater than 300 nm because significant absorption can occur at wave lengths below 200 nm as the result of the use of a photographic film negative or fillers used in the organic coating. As shown in U.S. Pat. Nos. 4,069,054, 4,026,705 and 4,250,053, various classes of sensitizing dyes can be used to alter the spectral response of aromatic sulfonium and aromatic iodonium salts. Those skilled in the art also have found that even if a particular dye is found to be effective as a sensitizer for a certain photoinitiator, for example, a triarylsulfonium salt, it may be entirely useless as a sensitizer with a different arylsulfonium salt because of the unpredictable nature of the spectral response for a particular arylsulfonium salt-dye combination.
The present invention is based on the discovery that particular sensitizing compounds, for example, aromatic hydrocarbons such as perylene, pyrene, anthracene, etc., can be used to sensitize arylacyldialkyl sulfonium salts of the formula, ##STR1## while aromatic ketones such as thioxanthone, chlorothioxanthone and benzophenone have been found to be effective as sensitizers for hydroxyaryldialkyl sulfonium salts of the formula, ##STR2## where R is a C.sub.(6-13) monovalent aromatic organic radicals or substituted C.sub.(6-13) monovalent aromatic organic radicals, R.sup.1 is a monovalent radical selected from hydrogen, C.sub.(1-8) alkyl and mixtures thereof, R.sup.2 is a C.sub.(1-13) monovalent organic radical, R.sup.3 is a monovalent radical selected from C.sub.(1-8) alkyl, hydroxy, C.sub.(1-8) alkoxy, halo and nitro, R.sup.4 is a C.sub.(1-8) alkyl radical, M is a metal or metalloid, Q is halogen and d is equal to 4-6.